1. Field of the Invention
This invention is an improved technique for producing optical fibers.
2. Description of the Prior Art
The many advantages of optical communications, both potential and realized, have stimulated significant efforts towards further development of this field of technology. Advantages that result from the use of visible radiation for the transmission of information were appreciated from the very inception of voice communication. Since these advantages are increased when the optical signal is in the form of coherent light, the discovery of the laser provided added impetus to the development of optical communication systems. Although the realization of a totally optical communication system still seems to be far in the future, the advantages of optical transmission alone are sufficiently impressive to warrant considerable effort in the development of optical transmission systems.
The major advantage in the use of visible and near infrared radiation for transmission purposes is associated with the increase in bandwidth over that available in simple electronic transmission systems. However, to utilize this increased bandwidth a medium capable of transmitting such optical signals must be developed. Basic electromagnetic theory indicates that light traversing a medium of index of refraction n.sub.1 will not be transmitted through an interface with a medium of index of refraction n.sub.2 if n.sub.1 is greater than n.sub.2 and if the angle made with the interface is less than arc cosine n.sub.1 /n.sub.2. Under such circumstances the light is contained in medium n.sub.2 and will be transmitted through this medium. This basic principle has led to the development of glass fibers for use as optical transmission lines. In this development a significant hurdle to be overcome is associated with the fabrication of fibers with optical loss low enough for practical applications. In the wavelength range from 4000 Angstroms to 1.5 microns the optical loss should be less than 50 db/km for short distance transmission, and less than approximately 4 db/km for long distance applications.
Additional problems are introduced when in order to effectively carry information the envisioned optical signal is in the form of optical pulses. Such pulses must be individually resolvable at the detecting end of the transmission line, as they were at the launching end. A number of phenomena, however, tend to broaden the pulses and consequently degrade the resolution. One of these phenomena is the frequency dispersion effect. As a result of this effect, light of different frequencies travels at different speeds within the fiber. Consequently, the different frequency components in an optical pulse of light are transmitted at different velocities arriving at the detector at different times, thereby broadening the pulse. The use of highly monochromatic light, e.g., from a laser, helps to alleviate the frequency dispersion problem.
In addition to frequency dispersion, there is a serious mode dispersion effect. This effect may be understood by considering the different paths that a given light ray may take as it traverses the optical fiber. It may, for example, proceed directly down the center of the fiber. On the other hand, it may reflect off the fiber walls numerous times as it traverses the fiber. Different parts of a given pulse may traverse the fiber in different modes and hence with different traversal times. These effects result in a general broadening of the pulse and in a consequent loss of pulse resolution. They are referred to by the term "mode dispersion."
Initial attempts to alleviate this problem involved the fabrication of single mode fibers. Such fibers will support only one specific mode and therefore do not display any mode dispersion. Technical difficulties were, however, encountered with single mode fibers. Launching an optical signal into a small diameter single mode fiber entails severe restraints on the coupling system between the source and the fiber. In addition, single mode fibers cannot efficiently transmit light produced by incoherent sources such as the common light-emitting diodes. Since such light sources are simpler and more economical than lasers, considerable interest has centered about multimode waveguides which can more efficiently transmit such light. In such waveguides the multimode dispersion effect must be reduced in order to maximize the information carrying capacity of the waveguide.
Mode dispersion in a multimode fiber may be minimized by utilizing a fiber that has a radially graded index of refraction. If such a fiber is properly designed, the velocity associated with light traveling near the fiber surface is greater than that associated with light traveling through the center of the fiber. Therefore, a higher velocity is associated with the long path length modes, which spend more time near the fiber surface, than with the short path length modes, which are generally confined to the fiber center. In this manner the transit times associated with the various modes is approximately equalized and the mode dispersion is minimized.
In an article by S. D. Personik in the Bell System Technical Journal, Volume 50, No. 3, March 1971, at page 843, an alternative technique for alleviating mode dispersion effects is suggested. Personik shows that while the pulse broadening associated with mode dispersion increases proportionately with the length of the fiber, enhanced and intentional mode conversion results in a broadening effect which is proportional only to the square root of the fiber length. Stimulated by this finding, numerous studies were made to determine the most effective techniques for enhancing mode conversion. One particular method involves the introduction of gradations in the index of refraction of the fiber along the longitudinal direction. However, in order to realize the benefits of intentional mode conversion while maintining radiation losses within tolerable limits the spatial period of such gradations must be between one and ten millimeters.
Optical fibers are drawn conventionally from preforms that are cylindrical in shape. The preforms are produced by depositing glass-forming materials onto a glass rod or within a hollow glass rod. The material accretes either inward toward, or outward away from, the axis of the cylinder being formed. Copending application Ser. No. 625,318 describes a different approach to fabricating preforms in which the preform cylinder is formed by accumulating material on the planar end of the preform rather than on the curved side. If the composition of the material which is being deposited is varied with time, longitudinal gradations, capable of effecting intentional mode mixing, are formed.